1. Field of the Invention
The present invention relates to a method of and an apparatus for stretching a thermoplastic polymer film or fibrous web monoaxially in a transverse direction or biaxially in transverse and longitudinal directions at the same time.
2. Prior Art
Various methods and apparatus have been employed in the art for transverse stretching or longitudinal and transverse stretching of a film or fibrous web. A typical example of such known methods and apparatus is a tentering system designed to grip the opposite selvages of a film or fibrous web and stretch the same monoaxially in a transverse direction or biaxially in longitudinal and transverse directions. Such system is however rather costly, space-consuming and complicated in structure. Another example known in the art is a tubular system which has been used frequently for biaxial stretching of a film. This system is relatively inexpensive to construct and takes up a relatively small space for installation, but it is not stable in operation and difficult to produce a film of good quality having a high magnification of stretch, and in particular the stretched film is difficult to heat treat upon stretching.
According to still another known system disclosed in each of U.S. Pat. Nos. 4,331,624 and 4,349,500, patented May 25 and Sept. 14, 1982, respectively, both assigned to the present assignees, a film is gripped along opposite selvages thereof between a pair of coplanar pulleys rotating in opposite directions and a pair of endless belt means trained arcuately around the pulleys, and then stretched by moving the gripped film selvages, respectively, along a pair of divergent substantially semicircular arcuate paths formed around the pulleys.
Although the disclosed system has performed satisfactorily in reducing all the drawbacks observed in the aforementioned two systems, it has not been found to be entirely suitable in application wherein the stretched film is required to have excellent properties. Since the film is reversed in direction of travel during movement of the gripped selvages along the arcuate paths, the non-supported central web portion is likely to take a short-cut or to be distorted upstream of its travelling path where the film is reversed, under longitudinal contractive stresses created during lateral stretching of the film. The stretched film is irregular in molecular orientation and strength. Further, it is difficult to stretch the film longitudinally and transversely at the same time, or transversely at a high magnification of stretch, solely by means of only one pair of pulleys.